Most electronic devices in use today include many “chips” interconnected to provide a specific functionality. The chips generally comprise a semiconductor die embedded in a package, wherein each die may comprise an integrated circuit formed by standard semiconductor fabrication processes. The semiconductor die typically has a series of bond pads, which are used to make electrical contact to the integrated circuit formed therein. The die is placed on a carrier or substrate that has electrical leads formed therein to correspond to the bond pads of the die. The die and the carrier are enclosed to protect the die from the environment. To increase the density of the integrated circuits, it is often desirable to stack dies such that two or more dies may be placed in the area of a single die on top of each other.
For example, high-end memory applications such as server applications or high-end mobile applications increasingly require higher memory densities. High memory density is traditionally obtained by either stacking packages on top of each other or by placing more memory components on the memory module. The density of some memory structures, however, may not be increased in this manner. For example, Double Data Rate II (DDRII) designs require low electrical parasitics of the overall package solution as well as a minimally small difference between dies to achieve high data rates of 400 Mbps to 800 Mbps, and higher. Stacking DDRII die adversely affects the operation because the upper chip has a longer net length to a defined merger point in the memory module than the bottom chip. The high-speed architecture also prohibits the placing of the memory components with a different distance to the module connector, because the stub length has to be kept constant for each component.
Another technical problem to be overcome in the case of stacking memory dies is the placement of the bond pads. Unlike logic dies that have bond pads along the periphery of the die, memory typically has bond pads in the center of the die. This arrangement restricts the accessibility of the bond pads when another die is placed on top of the bond pads of a lower die.